Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: March 4, 2011
Publication Date: May 1, 2011
Citation: Byrdwell, W.C. 2011. Multiple parallel mass spectrometry for lipid and vitamin D analysis. 8:225-299.
Interpretive Summary: This book chapter describes numerous ways in which multiple instruments that measure the masses of molecules (mass spectrometers) can be used in parallel to provide much more information about samples than single instruments used alone. The techniques discussed include ‘dual parallel mass spectrometry’ in which two mass spectrometers are used in parallel, and ‘triple parallel mass spectrometry’ in which three mass spectrometers, operating in different modes, are used in parallel. Experiments are also described in which two systems for separation of molecules (liquid chromatographs) are used in combination to analyze complex samples. An extract of all polar and non-polar lipids are injected on one liquid chromatograph, and while some molecules are retained on the chromatography column, others are not. The molecules that are not retained on the first column are switched, by an electronic valve, over to another chromatography column tailored just for the separation of those molecules. The output from each of the liquid chromatographs is sent to a mass spectrometer for detection. By using this arrangement, both the polar and non-polar molecules in a complex sample can be separated and identified independently on two differed systems in a column-switching arrangement.
Liquid chromatography (LC) coupled to mass spectrometry (MS) has become the method of choice for analysis of complex lipid samples. Two types of ionization sources have emerged as the most commonly used to couple LC to MS: atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). Some classes of lipids are more amenable to analysis by one type of ionization versus another, and the optimal conditions for each vary, such as gas flow, temperature, and voltage. Thus, no one type of ionization is ideal for all classes of lipids, and different ionization modes may be suitable for different lipid classes within the same mixture. Furthermore, there are different MS scan modes that provide different types of information, such as full-scan mode, selected ion monitoring (SIM), selected reaction monitoring (SRM), product ion scanning, precursor ion scanning, and neutral loss scanning. Unfortunately, the duty cycle of the mass spectrometer is finite, which limits the number of experiments that can be performed in a single run. Thus, there is no single LC-MS technique that is ideal for analysis of all lipid classes.
This chapter describes LC-MS methods that employ two or three mass spectrometers connected in parallel as detectors for a single chromatographic system (LC1/MS2 or LC1/MS3) for analysis of molecules using APCI and ESI ionization modes in a single run, or that employ two liquid chromatographs and two mass spectrometers in a column switching arrangement (LC2/MS2) for analysis of polar and non-polar lipids on different LC systems from a single injection. These data demonstrate how using multiple MS systems can reduce or eliminate the need for repeat injections and repetitive experiments. The data demonstrate how the shortcomings of any single instrument are overcome by employing mass spectrometers that acquire complementary data from the same analytes, and demonstrate that such an arrangement provides a greater degree of confidence in both qualitative and quantitative results.